Acoustic device



T. J. POPE ACOUSTIC DEVICE Filed Feb. ll, 1939 2 Sheets-Sheet 1 v r on sAx/s (MEcHAN/:AL Ax/s) x ORA Ax/s (ELECTRICAL Axis) l/'l 2l INI/ENTOR ByT. J. POPE @4M/em 6. fdm.

A from/EV May 20, 1941. T, 1 POPE 2,242,755

ACOUSTIC DEVICE Filed Feb. l1, 1939 2 Sheets-Sheet 2 E" v- 30 /9 3/ 32l33 /N VEA/TOR 7. J. POPE A TTORNY Patented May 20, 194i t ACOUSTICDEVICE Thomas J. Pope, Brooklyn, N. Y., assigner to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation oi' New YorkApplication February 11, 193i), Serial No. 255,817

(c1. ris- 110) 8 Claims.

This invention relates to acoustic devices and more particularly totelephone receivers having a diaphragm actuated by a piezoelectricelement.-

One object of this invention is to improve the efficiency and responsecharacteristic of electroacoustic devices.

Another object of this invention'is to simplify.

the structure and to reduce the weight of telephone receivers.

Still another object of this invention is to enable the use ofrelatively thin, large area piezo' -electric crystals in electroacousticdevices whereby a small electrical impedance is obtained.

A further object of this invention is to. decrease the load or impedanceupon the driving point or points of a piezoelectric element in anelectroacoustic device.

A still further object of this invention is to obtain a relativelysimple mode of movement or vibration of a piezoelectric actuatingelement in telephone receivers. s

of this invention, a portion of the casing being broken away to show theinternal structure;

Fig. 2 is a diametral sectional view of the device shown in Fig. 1; l

Fig. 3 is a detail view in section of a composite piezoelectric elementconstructed in accordance with this invention;

Fig. 4 is a view in perspective illustrating one manner of obtainingcrystal slabs incorporated In one illustrative embodiment of thisinvention, a telephone receiver comprises a diaphragm v having aradiating portionadapted to reciprocate bodily, analogous to a piston,and a piezoelectric element for actuating the diaphragm, which elementmay be partially restrained or substantially unrestrained except byvirtue of its connection to the'diaphragm.

In accordance with one feature of this invention, the piezoelectricelement comprises a pair of relatively thin rectangular crystal slabssecured together in face-to-f-ace relation. The two crystal slabs havetheir axes so oriented that diagonally opposite corners of thepiezoelectric element tend to move in the same direction in response toelectrostatic forces produced in the crystal slabs.

In accordance with another feature of this invention, one pair ofdiagonally opposite corners of the composite piezoelectric element isconnected to the diaphragm, for example, to diametrically oppositepoints of the bodily vibratile l portion thereof.

'and fully from the following detailed description Y with reference tothe accompanying drawings wherein:

Fig. 1 is a. perspective view of a piezoelectric acoustic deviceillustrative of one embodiment Fig. 'l is a perspective View of stillanother illustrative embodiment of this invent-loma portion of thecasing being broken away to show the internal structure; and d Fig. ,8is a graph showing a typical response characteristic of a device of theconstruction i1- lustrated in Fig. 7;

Referring now to the drawings, the piezoelectric device shown in Fig. 1comprises a cup-shaped -casing I0, for example, of molded insulatingma-` terial. having at one' end an annular internal shoulder I I and acentrally apertured cap or earpiece I2 threaded to the casing I0. Seatedon the annular shoulder Il is an annular support or spacer I3, whichmay' be of metal. Seated in turn upon the spacer or support I3 andclamped thereagalnst by the cap or ear-piece I2 is a diaphragin, whichmay be made in a single piece and preferably is of a light-weight metalsuch as duralumin.

As shown clearly in Fig. 2, the diaphragm comprises a il'at ilexibleperipheral or marginal portion It, a part of which is held between thecap or ear-piece I2 and the support or spacer I3, and a bodily vibratileportion in line with the opening in the cap or ear-piece I2. Thevibratile portion includes a central dished or concavo-convex porofdiaphragms may be used.

A u-shaped bracket or bridge member la is aiilxed at its ends, as bywelding. to diametrcally opposite areas of the support or spacer I3 andsupports a composite, rectangular' or square piezoelectric elementdesignated generally as .20. As shown clearly in Figs. 2 and 3, thepiezoelectric element 20 comprises a pair of crystal plates or slabs 2Ia and 2 Ib, for example, 1.0 inch square and 1/4th inch thick, eachcrystal having upon the opposite faces thereof a. coating 22 ofelectrically conductive material. For example, the coatings 22 may be ofmetal, e. g. gold evaporated upon the crystals in vacuum, or metal. foilcemented to the crystal slabs, or they may be of graphite 'produced byapplying to the slabs a colloidal graphite suspension, knowncommercially mented to the crystal slabs and in intimate contact withthe coatings 22.

The crystal slabs 2| may be cut from a mother` crystal 26, for. exampleof Rochelle salt, having its characteristic axes as shown in Fig. 4. Aswill be apparent from this ligure, they faces of the crystal slabs areparallel to the Y or B and Z or C axes and normal to the X or A axis.

As is known in. the art, piezoelectric crystals possess the property ofexpanding and contracting when subjected to electrostatic `stresses and,conversely, of producing potential differences in accordance withmechanical stressing of the crystals. .The crystals 2Ia and 2lb areassociated so that the axes of maximum expansion and contraction arealong the diagonals of the crystal slabs, as shown by the arrows in Fig.5, and so that each of these axes of one crystal slab is at an angle, e.g., at right angles, to the corresponding axis of the other crystal.That is to say, the crystals are so orientated that in response to anelectrostatic stress produced therein, when the v crystal slab 2mexpands along the diagonal AB and simultaneously contracts along thediagonal CD, the crystal slab 2lb will contract along the diagonal C'D'and expand along the axis AB.

This relationship may be obtained, for example, by applying thepotentials between the outer conductive layers, connected together, andthe inner conductive layers, connected together, and having the crystalslabs associated so -that 'the Y or B and Z or C axes of one areparallel to the corresponding axis of the other. Alternatively, thisrelationship may be obtained by composite crystal will move upwardly andthe corners CB' and DA will move downwardly. Y

Hence, the composite crystal willtransform potential variations. such asthose corresponding to speech and music frequencies, into mechanicalvibrations at the corners of .the crystal. Conversely, of course,vibration of the corners of the crystal may be transformed into variablepotentials.

As shown in Figs. l and 2, the composite crystal 20 is securely aixed,as by a suitable adhesive,l along one diagonal to the bridge member orbracket I9. The corners at the ends of the other diagonal are connectedto diametrically opposite points on the annular portion I8 of thediaphragm, as by balsa wood blocks 21 cemented to the crystal and thediaphragm.

It will be noted that because of the application of driving forces'totwo points on the diaphragm, the diaphragmimpedance and the air volumestiffness into which the diaphragm works are substantially halved sothat relatively thin crystals may be used. The crystals, being of largearea, therefore, have a relatively small electrical impedance. Thedecreased load on each driving point of the crystal assures a simplemode of vibration throughout a. wide band of frequencies.

Finally, it is apparent that acoustic devices constructed in accordancewith this invention bridge member I9. The member 29 preventsdisplacement of the crystal and restrains vibration thereof as a whole.4Two diagonally opposite corners of the crystal, therefore, are free andunrestrained. Ifdesired, however, these two corners may be connected tothe diaphragm by lever systems, not shown, so that they impart to thediaphragm forces in the same direction, at any instant, as the other twocorners.

1f desired, the response characteristic of the device may be controlledby coupling an acoustic orientatingl the two slabs so that the Y and Zimpedance to the vibratory element.

'For ex-l ample, as shown in Fig. 6, a plate or disc 30 having anaperture 3|, or a plurality of aperturesI therein, the aperture orapertures being covered with an acoustic resistancematerial 32, such assilk, may be provided behind the crystal 20. The plate or disc 30 may beseated on an internal shoulder 33 in the casing and held thereagainst bythe bracket member or bridge I9. The paraml eters of the disc and plate,theapertures theresurface of the crystal 2 Ib must be insulated from thelower surface of the crystal 2 la. l Y

As will be apparent from Fig. 5, when the polarity of the appliedpotential is such as to cause expansion and' contraction ofl the crystalslabs In the directions indicated by the arrows, theY composite crystalwill bend so that the corners AC and BD' thereof move downwardly and thecorners CB and DA' move upwardly. When the polarity of the potential' isreversed, the. direc- 'v tions oi expansion and contraction will be reinand the chambers formed thereby may be made such that the acousticimpedances dened thereby alter the response characteristic of the devicein any desired manner. For example, the parameters may be such that theacoustic impedances introduced into the electroacoustic sysitem of thedevice suppress peaks in the response ticular frequencies.

characteristics or increase the response at par- In the acoustic deviceshown in Fig. 7, which is of the inertia type, the crystal, diaphragmand casing are the same asin the embodiment shown in Figs. 1 and 2, twodiagonally opposite corners of the crystal 20 being connected todiametrically opposite points on the annular portion I8 of the diaphragmby the blocks 21. However, as will be seen, the composite crystal 20 issupported solelyv by the diaphragm. Two diagonally opposite corners ofthe crystal 2B may be free as shown, or, if desired, they may beconnected to the diaphragm through lever systems as described heretoforein connection with Fig. 6 so that at any instant, forces of the samedirection are applied to four equally spaced points of the diaphragm.

A typical response characteristic for a receiver of the constructionshown in Fig, 'l is illustrated in Fig. 8 from whichit will be apparentthat a high average response is obtained throughout the range offrequencies between about 400 and 2,500 cycles per second. The peaksoccurring at the extremities of this frequency band may be substantiallysuppressed and the frequency range extended by the use of an acousticimpedance system as shown in Fig. 6 and described heretofore.Alternatively, these peaks may be compensated for by appropriate designof the electrical system connected to the piezoelectric device.

It will be understood, of course, that an acoustic impedance system maybe utilized also in the construction shown in Figs. 1 and 2 and that theoutput of a system including devices of the construction shown in Figs.1 and 2 and Fig. 6 may be controlled by design of the electrical systemconnected to the piezoelectric device.

It will be understood also that although several specic embodiments ofthis invention have beenshown and described, various modifications maybe made therein without departing from the scope and spirit of thisinvention as defined in the appended claims,

What is claimed is: i

1. A piezoelectric device comprising a vibratory member, a.piezoelectric element including a pair of piezoelectric crystals securedtogether in face-to-face relation and having their axes orientated sothat diagonally opposite points of said element move in the samedirection in response to electrostatic forces produced therein, twodiagonally opposite points of said element being connected to saidvibratory member, and means fixing said element against deflection alonga line passing through two other diagonally opposite corners.

2. An acoustic device comprising a diaphr-agm, a composite piezoelectricelement including a pair of crystal plates secured together infaceto-face relation, said plates being orientated so that each of the Yand Z axes of one is parallel to the corresponding axis of the other,means connecting two points on an axis of contraction and expansion ofsaid element to said diaphragm, and rigid means aiiixed to said elementalong the other axis of expansion and contraction thereof.

3. A piezoelectric device comprising a composite piezoelectric elementincluding two substantially square crystal plates secured together inface-to-face relation, each of the'axes of expension and contraction ofone plate being at right angles to the corresponding axis 4of the other,a vibratory member connected to one pair of diagonally opposite cornersof said piezoelectric element, and a rigid member fixing said elementagainst deflection along a line passing through the other pair ofdiagonally opposite corners thereof.

4. An acoustic device comprising a diaphragm having a rigid centralportion, a substantially square composite piezoelectric element coaxialwith said diaphragm, said element including a pair of substantiallysquare Rochelle salt slabs securedv together in face-to-,face relationwith their electrical axes coincident and having diagonal axes ofexpansion and contraction, each of the axes of expansion and contractionof one being at right angles tothe corresponding axis of the other,means connecting two diagonally opposite corners of said element `todiametrically opposite points at the margin of said rigid centralportion, and means constraining the central portion of said elementagainst movement.

5. A piezoelectric device comprising a lrectangular composite elementincluding a pair of piezoelectric plates secured togetherin face-tofacerelation and having their characteristic axes orientated so thatdiagonally opposite corners of said element move in the same directionsin response to electrical potentials impressed upon said element, meansrigidly flxing said element along a line connecting one pair ofdiagonally opposite corners of said element, a vibra- .at right anglesto each other, rigid means ex` tending along one of the axes ofexpansion and contraction of 'one of said plates and aiiixed to said oneplate along said one axis thereof, and

vibratory means connected to spaced points on the axis of said elementnormal to said one axis of expansion and contraction.Y

7. An acoustic device comprising .'a support, a

diaphragm having a portion seated on said support and a centralvibratile portion, `a bridge member carried by said support, apiezoelectric plate supported by said bridge member and fixedA theretoalong one of its axes of expansion and contraction, and means connectingspaced points of said plate to said vibratile portion.

l8. An acoustic device comprising a support, a diaphragm having acentral vibratile portion, an outer supporting portion seated on saidsupport and an intermediate portion, abridge member aiiixed to saidsupport, a piezoelectric element including a pair of crystal slabssecured together in face-to-face relation with theY and Z axes of oneeach parallel to the corresponding axis of the other, whereby the axesof deection of 'said elementare at right angles to each other,

said-.bridge member being secured to said element along one axis ofdeflection thereof, and means connecting spaced points on the other axisof deflection of said element to diametrically opposite points of saidintermediate Portion.

. THOMAS J. POPE.'

